1. Field of the Invention
The present invention relates to an image forming apparatus comprising a number of light emitting elements capable of emitting light beam onto a photosensitive material based on given image information, a number of optical fibers extending between a light source and the light emitting elements for guiding light irradiated from the light source to the light emitting elements, each of the optical fibers having a light receiving end for receiving the irradiated light, and a cooling device for cooling the receiving ends of the optical fibers with gas stream.
2. Background of the Related Art
With the image forming apparatus of the above-noted type, a number of optical fibers of a small diameter are bundled together to be used as a kind of flexible light guiding means. And, the irradiated light from the light source is intensified by means of a converging means such as a convex lens and then this intensified light is caused to enter a small flat terminal face of the bundled optical fibers, i.e. the light receiving ends of the fibers.
Therefore, the bundled optical fibers, in particular, their light receiving ends, tend to be overheated in the course of transmission of the light through the optical fibers. And, especially when the optical fibers are made of resin (e.g. a typical commercially available optical fiber made of resin has a manufacturer""s guaranteed heat resistant value of 70xc2x0 C. or slightly higher as its maximum allowable temperature), the light receiving end of the fiber may be deformed by the heat, thus impaired in its flatness, leading to abnormal reduction in the light incident efficiency.
Then, as means for protecting the optical fiber per se against the heat, it has been proposed to provide cooling means for cooling the light receiving end of the optical fiber with gas stream such as air stream. As this cooling means, there is used a blower fan which is rotatably driven by means of an electric motor. This blower fan is operated while the light source is ON and is stopped simultaneously with turning OFF of the light source. In this manner, the convention has adopted a simple operational scheme synchronizing the activation/deactivation of the fan with ON/OFF of the light source.
Aside from the above, with the conventional image forming apparatus described above, there often occurs another phenomenon of dust (e.g. fibrous dust) being accumulated on the light receiving end of the optical fibers. In such case, even when the apparatus includes a cooling device such as above, as soon as the light source is switched ON, some of the accumulated dust may be carbonized quickly by the heat and stuck on the light receiving end of the optical fiber before it can be effectively blown away by the air stream from the cooling fan. Then, there occurs local reduction in the transmittivity of the optical fibers, again resulting in deterioration in the quality of image to be formed by the image forming apparatus. Moreover, if the optical fiber is made of resin, the reduction in transmittivity due to the carbonized dust described above will render the light receiving end of the optical fiber even more vulnerable to the heat of the irradiated light from the light source, so that the local melting deformation thereof will more likely occur. Then, in the worst case, replacement of the entire image forming apparatus (e.g. the entire digital exposing head thereof) may become necessary.
One conceivable cause of the above-described problem is as follows. The amount of light irradiated from the light source rises quickly upon turning ON of the light source. On the other hand, the rotational speed of the electric motor constituting the cooling device does not rise so quickly. Hence, there is a certain amount of time lag before the cooling effect of the cooling device asserts itself on the light receiving end of the optical fiber. During this short time lag, the overheating and subsequent carbonization of accumulated dust by the irradiated light occurs on the light receiving end of the optical fiber. Once the accumulation and subsequent carbonization of the dust on the terminal face of the optical fiber has developed to a certain degree, even if the cooling device is beginning to provide its full cooling effect after lapse of the time lag, this may be too late, so that there will be no recovery in the reduced light transmittivity at the light receiving end of the optical fiber.
The present invention attends to the above-described state of the art. The primary object of the invention is to provide an image forming apparatus which can avoid the above-described problem of reduction in the light transmittivity of the optical fibers by preventing dust, when accumulated on the light receiving end of the optical fiber, from becoming carbonized by the heat of the irradiated light from the light source.
For accomplishing the above-noted object, an image forming apparatus according to the present invention comprises:
a number of light emitting elements capable of emitting light beam onto a photosensitive material based on given image information;
a number of optical fibers extending between a light source and the light emitting elements for guiding light irradiated from the light source to the light emitting elements, each of the optical fibers having a light receiving end for receiving the irradiated light;
a cooling blower device for cooling the receiving ends of the optical fibers with gas stream; and
a controller operable to execute a pre-blowing/cooling step of operating the cooling blower device for a predetermined period before the light source is turned ON in response to an ON instruction.
With the above-described feature, according to the image forming apparatus of the invention, the light source is not immediately turned ON in response to an ON instruction. Instead, the illumination of the light source is allowed only when the required condition of execution of the pre-blowing/cooling step has been met, that is, only after the cooling blower device has been operated for a predetermined time period. That is, for the predetermined time period between input of the ON instruction to the light source and the actual illumination of the light source, the preliminary operation of the cooling blower device is effected. And, during this preliminary operation, dust accumulated on the light receiving end of the optical fibers will be blown away by the cooling gas current fed from the cooling blower device to the light receiving end of the optical fibers, so that the blown dust will be moved to a surface or into tissue of a dust collecting filter which is disposed within a ventilation hole formed between a housing of the image forming apparatus and the outside. Accordingly, it is possible to prevent the dust from being carbonized by the heat of the irradiated light and permanently stuck on the light receiving ends of the optical fibers. Consequently, it becomes possible to avoid deterioration in the light transmittivity of the optical fibers and corresponding deterioration in the quality of image to be formed by the image forming apparatus and to avoid also fusing deformation of the light receiving ends of the optical fibers and the resultant replacement of the entire digital exposure head of the apparatus can be avoided also.
According to a further characterizing feature of the present invention, the apparatus further comprises:
light modulating means for modulating wavelength of the irradiated light, the light modulating means being a rotary filter unit interposed between the light source and the optical fibers, the rotary filter unit including a plurality of color filters arranged side by side along a peripheral direction of the filter unit and having cut wavelength ranges different from each other; and
rotation detecting means for determining whether a rotational speed of the rotary filter unit is within a predetermined range or not;
wherein the controller allows illumination of the light source upon determination by the rotation detecting means of the rotational speed of the rotary filter unit being within the predetermined range, after execution of the pre-cooling/blowing step.
With the above construction, the rotary filter unit mounts a plurality of color filters disposed along the rotary peripheral direction of the unit and having different colors such as R (red), G (green), B (blue), etc. In operation, as this rotary filter is rotated at a high speed, the color filters thereof come in and out of a path of the irradiated light which is a white light, thereby changing the color components of the light one after another to be emitted from the light emitting elements onto the photosensitive material such as a print paper. In the course of this, each light emitting element of the exposing engine is turned ON/OFF momentarily according to given image information so as to transmit the light beam of each color component through the rotary filter onto the photosensitive material.
That is to say, according to the above-described construction, the illumination of the light source is allowed after the rotational speed of the rotary filter unit has reached the predetermined range. Therefore, it is possible to avoid not only damage of the optical fibers by overheating, but also damage of the rotary filter unit by overheating with the irradiated light from the light source when the rotary filter unit is not rotated at all or rotated at a lower speed.
According to a still further feature of the present invention, the controller is operable also to execute a post-blowing/cooling step of operating the cooling blower device for a predetermined period after the light source is turned OFF in response to an OFF instruction.
With this construction, when an operator inputs an OFF instruction, the light source is turned OFF immediately. On the other hand, the cooling blower device is not stopped immediately, but it continues to operate for a predetermined time period until the light receiving ends of the optical fibers have been cooled sufficiently (e.g. down to the room temperature). And, during this continued operation too, it is possible to prevent any dust present around the apparatus from being accumulated on the light receiving ends of the optical fibers. As a result, this construction can avoid such inconvenience of dust accumulated on the optical fiber from being carbonized and stuck on its light receiving end by the heat remaining in the fibers after the light source has been turned OFF.
According to a still further feature of the present invention, the apparatus further comprises:
cooling device monitoring means for monitoring the operational condition of the cooling blower device; and
alarming means for reporting monitored result of the monitoring means to an operator when the operational condition of the cooling blower device is determined not normal.
With this construction, in response to alarm from the alarming means indicating any abnormality in the operational condition of the cooling blower device, the operator can manually operate an OFF switch of the light source. As the result, it is possible to minimize the damage of the light receiving ends of the optical fibers by the heat of the irradiated light.
According to a still further feature of the present invention, the cooling blower device comprises a fan and the operational condition of the cooling blower includes a rotational condition of the fan.
The cooling device monitoring means for monitoring the rotational condition of the fan can be realized by a DC motor having a sensor for detecting reduction in the rotational speed of the fan due to e.g. presence of some foreign substance by means of an eddy current generated in association therewith.
According to a still further feature of the present invention, the operational condition of the cooling blower device includes a positional relationship between the cooling blower device and the light receiving ends of the optical fibers.
The cooling device monitoring means operable to determine the positional relationship between the cooling blower device and the light receiving end of the optical fibers can be realized by a magnetic proximity switch attached to e.g. an air nozzle of the cooling blower device such that its electromagnetic circuit is closed only when the air nozzle has a proper positional relationship relative to the light receiving end of the optical fibers.
Further and other features and advantages of the present invention will become apparent from the following detailed discussion of the preferred embodiments with reference to the accompanying drawings.